In the control of internal combustion engines, the conventional practice utilizes electronic control units having volatile and non-volatile memory, input and output driver circuitry, and a processor capable of executing a stored instruction set, to control the various functions of the engine and its associated systems. A particular electronic control unit communicates with numerous sensors, actuators, and other electronic control units to control various functions, which may include various aspects of fuel delivery, transmission control, turbocharger control, or many other functions.
A turbocharger consists of a turbine and a compressor. The pressure of the engine exhaust gases causes the turbine to spin. The turbine drives the compressor, which is typically mounted on the same shaft. The spinning compressor creates turbo boost pressure which develops increased power during combustion.
A variable geometry turbocharger has movable components in addition to the rotor group. These movable components can change the turbocharger geometry by changing the area or areas in the turbine stage through which exhaust gases from the engine flow, and/or changing the angle at which the exhaust gases enter or leave the turbine. Depending upon the turbocharger geometry, the turbocharger supplies varying amounts of turbo boost pressure to the engine. The variable geometry turbocharger may be electronically controlled to vary the amount of turbo boost pressure based on various operating conditions.
In a variable geometry turbocharger, the turbine housing is oversized for an engine, and the exhaust gas flow is choked down to the desired level. There are several designs for the variable geometry turbocharger. In one design, a variable inlet nozzle has a cascade of movable vanes which are pivotable to change the area and angle at which the exhaust gas flow enters the turbine wheel. In another design, the turbocharger has a movable side wall which varies the effective cross-sectional area of the turbine housing.
A conventional variable geometry turbocharger control system utilizes an electronic controller having a boost map stored therein. The boost map contains the optimum boost for an engine as a function of engine operating conditions. The controller monitors the engine operating conditions using sensors, and determines the desired boost from the boost map. Turbocharger geometry is incrementally adjusted based on the desired boost pressure obtained from the boost map by actuating or deactuating a pneumatic cylinder that drives a control arm on the variable geometry turbocharger. Moving the control arm causes the turbocharger geometry to change. Typically, the pneumatic cylinder is driven in an open loop fashion in accordance with the boost map.
A primary disadvantage associated with existing variable geometry turbocharger control systems is the fact that turbo boost pressure has a slow response time to incremental changes in turbocharger geometry. The slow response of turbo boost pressure is due in part to the response characteristics of the pneumatic cylinder and associated open loop control techniques. Because the optimum boost from the boost map varies continuously with varying engine operating conditions, the slow response time of the turbo boost pressure to the incremental changes in turbocharger geometry makes it difficult to obtain precise control of the turbocharger. This slow response time may sometimes render some of the emissions and driveability benefits of the variable geometry turbocharger unachievable.